Sound management of Spent Potliner (waste from the production of primary aluminum by the electrolytic process known as the Hall-Heroult process) has been an issue of concern for governmental regulatory agencies and industry alike. Spent Potliner is known to be contaminated with large amounts of harzardous materials (cyanide and fluorine). Heating value for this waste varies from 3000 to 5000 BTU per pound. The quantity of Spent Potliner generated and discarded annually in the United States alone, has exceeded 200 thousand tons. In addition, over 1,200 thounsand tons are presently found in recoverable storage, awaiting a final destiny, and much more yet festering in landfills. Because of its high concentrations of fluorine and cyanide, Spent Potliner was recently listed as "hazardous" (EPA hazardous waste # K088, Sep. 13, 1988).
Several management alternatives to land-based disposal of Spent Potliner have been suggested. Among them, some are considered "disposal techniques" such as using Spent Potliner as a fluorspar substitute in iron-melting and steel making, or as fuel in cement manufacture or fluidized bed combustion. Others have "recovery techniques" such as recovery of cryolite, molten salt recovery of chemical and energy values, pyrohydrolysis, and pyrosulfolysis. However, the complexity, cost disposal of residuals, and engineering problems of most of these processes make them economically unacceptable. Accordingly, landfilling and stockpiling are still the only practical and feasible alternative.
The most promising solution to management of this waste stream is fluidized bed combustion. During the last decade, fluidized bed combustion has been widely adopted for burning high-sulfur fuel and has gained commercial acceptance for the disposal of a growing number of hazardous materials. The advantages of this process are well established: high turbulence and residence time of the waste in the combustion chamber allow complete combustion at a moderate temperature (850.degree. C).
A number of attempts at incinerating Spent Potliner by fluidized bed combustion have been made. However, those systems exhibited extreme operating difficulties primarily due to the formation of clinkers and agglomeration of the ash, off-gas (HF) emission control, ash-fluoride leachate control, and heavy metal leachate control. Agglomeration causes segregation and defluidization, consequently shutdown of the process. Emission and leachate control directly influence the short and long-term environmental consequences from the process and therefore the overall process feasibility.
Management of Spent Potliner has been subject to U.S. Pat. #2,858,198 published in Oct. 28, 1958. This invention involves recovery of fluorine from Spent Potliner by distillation. In that disclosure, coarse pieces of Spent Potliner are heated in a furnace to over 1000.degree. C. under sub-atmospheric pressure, thereby volatilizing fluorides. Processes involving fluidized beds have also been subject to many U.S. patents, e.g. combustion of sulfur-containing fuel by Pat. #4,103,646 and #4,579,070; recovery of sulfur from native ores by volatilization of the free sulfur from the ore (U.S. Pat. No. 3,102,792); desublimation of gaseous aluminum chloride to solid form (patents #3,930,800 and #4,334,898); sublimation of phosphoric acid anhydride (U.S. Pat. No. 3,077,382). None of these processes disclose an application of fluidized bed combustion technology to Spent Potliner, nor the introduction of property additives to control agglomeration. U.S. Pat. No. 4,763,585 does address the fluidized bed combustion of Spent Potliner. In that disclosure a physical coating is applied to the Spent Potliner in order to reduce the stickiness of the particle at operating temperatures. That is achieved exclusively by physical means by coating tacky particles with any of various inert fine powders to reduce the stickiness; almost any dirt will serve.
In the process revealed by this application, ash chemistry is regulated--with additive--for three purposes: 1. To chemically create a non-sticky compound, within and on the surface of Spent Potliner and ash particles, that does not display an adhesive tendency and form agglomerates. 2. To reduce to a minimum the leachate conscentration of fluoride anion and metal cations from ash samples removed from the process and subjected to standard leach procedures. 3. To minimize hydrogen fluoride emissions in-situ prior to subsequent off-gas treatment by chemically reacting HF out of the gas stream.
The primary cause of agglomerate formation while firing fuel blends that include Spent Potliner is due to the composition of Spent Potliner itself. Alkali-halide compounds in Spent Potliner--thought of as impurities--form a low-melting eutectic at fluidized bed combustion temperatures that behaves as a glue and causes a tendency for the ash to agglomerate.
In addition to the problem of agglomeration, control of both off-gas emissions and residual-ash leachate concentrations must be achieved to satisfy regulatory constraints.
A significant contribution to the art would be a complete systems-approach Spent Potliner management alternative, that would be a safe, economical, technically feasible, and an environmentally acceptable process. Such a process is provided by this invention.